Excessive use of conventional fossil fuels has resulted in environmental pollution and an energy crisis. As a result, government agencies, enterprises and researchers all over the world are increasingly concerned with discovering sustainable energy sources and the development of energy conversion devices. A fuel cell is an energy conversion device that converts the chemical energy of a fuel, when combined with an oxidant, directly into electricity with merits such as high efficiency of energy conversion, non-pollution and many others. A proton exchange membrane fuel cell (PEMFC) operates at relatively low temperature and has a compact structure.
Among various fuel cells, PEMFCs are particularly promising for use as a back-up power station, stationary power generation, or as a portable power supply for electronics and vehicles. However, the storage and transportation of hydrogen, the major fuel for PEMFC, is still very difficult. As a relative and strong candidate, both direct methanol fuel cells (DMFC) and direct formic acid fuel cells (DFAFC) show significant advantages over PEMFCs in energy supply. Recently, continuous efforts have been put into the development of DFAFCs, due to higher theoretical open circuit potential, lower fuel crossover and many other advantages over the direct methanol fuel cell.
Two key issues inhibit the commercialization of DFAFC: the first being low efficiency and the second being the poor stability of its catalysts. Platinum (Pt) is major fuel cell catalyst component. However, carbon monoxide (CO) from the oxidation of formic acid can easily poison (contaminate) the platinum and cause it to lose its catalytic function. Therefore, palladium (Pd) based compounds are often used as an anodic catalyst in DFAFC. Unfortunately, a Pd-based composite catalyst may also display poor stability if it suffers “the poisoning effect” from the carbonaceous intermediates generated during the reaction.
The formic acid on the surface of platinum is oxidized to CO2 via a dual-path mechanism. A direct dehydrogenation path generates CO2 which is not harmful to the catalyst, while an indirect dehydration path generates CO which is toxic to the catalyst. One solution is to incorporate some atoms of bismuth or lead onto the surface of the platinum to improve its effectiveness in formic acid oxidation and to guide the oxidation toward the more desirable direct dehydrogenation path. However, absorbed atoms are very unstable on the surface of platinum. Hence, this type of surface modification to improve the catalyst has little value in practical applications.
In order to increase the utilization of platinum, amorphous carbon (carbon black or carbon fiber) may be used in the catalysts for fuel cells as an inexpensive means of support. However, carbon support is unstable during the electro-oxidation reaction and so the utilization of platinum is still not high enough to satisfy commercial applications. A need exists to find a new method of preparing a catalyst with low Pt loading, strong toxic resistance, and long service life for direct formic acid fuel cells.
Metal alloys having a certain composition can be turned into uniform nanoporous metal materials by electrochemical etching. Uniform nanoporous metal materials have a high specific surface area and an adjustable structure. Uniform nanoporous metal materials are able to serve as structural support in electrocatalysts due to following distinctive characteristics: 1) large specific surface area due to three-dimensional nanoporous structure; 2) superior conductivity; and 3) strong corrosion resistance. Karl Sieradzki and Roger C. Newman reported a method of forming porous metal structure by removing the silver via electrochemical etching (Karl Sieradzki, Roger C. Newman “Micro- and Nano-porous Metallic Structures” U.S. Pat. No. 4,977,038, Dec. 11, 1990). In 2004, inventor of the instant patent application acquired a US patent on a method of forming nanoporous membranes with high specific surface area by etching commercial metal alloy membranes (Jonah Erlebacher, Yi Ding “Method of Forming Nanoporous Membranes” U.S. Pat. No. 6,805,972). In the same year, the inventor of the instant patent application acquired a worldwide patent on a method of plating nanoporous metal membranes by an electroless plating process of reducing precious metal ions with hydrazine vapor (Jonah Erlebacher, Yi Ding “Method of Plating Metal Leafs and Metal Membranes”). In the instant invention the catalysts were fabricated via a successive deposition of thin platinum and gold layers onto the surface of nanoporous gold membranes with monoatomic layer precision. The techniques involved in this procedure are Under Potential Deposition and replacement reaction.
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